Prolactin actions in the mouse mediobasal hypothalamus (#242)

The mediobasal hypothalamus regulates
pituitary function through the actions of hypophyseal neurons. Prolactin, for
example, is regulated by release of dopamine from hypothalamic neurons that
project to the median eminence (ME). The ME transports dopamine via portal blood
to the anterior pituitary, where it inhibits release of prolactin. Prolactin
itself is able to mediate its own regulation by signalling back on hypothalamic
cells through the phosphorylation of Signal Transducer and Activator of
Transcription (STAT)5. Our previous data has shown prolactin induced
phospho-STAT5 in the nuclei of many cells of the mouse mediobasal hypothalamus.
These experiments also show both nuclear and granular phospho-STAT5expression
in the ME. The aim of this current study was
to examine the time course of prolactin-induced phospho-STAT5 in the
hypothalamus compared to the ME of the male mouse.

Male C57BL/6 mice were treated with bromocriptine over 24 h in order
to block endogenous prolactin release. Intraperitoneal injections of 2mg/kg
ovine prolactin were administered over 2, 5, 20 or 60 min. Animals were
euthanised with pentobarbital and perfused with 4% paraformaldehyde. Brains
were excised and cryostat sections prepared. Phospho-STAT5 staining was
visualised through immunohistochemical techniques. Phospho-STAT5 in the arcuate
nucleus and ME was counted, where nuclear, or analysed using densometric
measures where apparent in a punctate form. The results show a temporal change
in the nuclear staining observed in the arcuate nucleus, with immediate
responses at 2 min (p<0.005), increasing by four-fold at 60 min. In the ME,
the response visible at 2 min was three-fold the saline response (p<0.05),
but remained at this level across the time course. This data demonstrates the
responses of the ME are temporally distinct to that observed in the arcuate
nucleus suggesting a possible functional separation